What does this mean

Problem $2\mathrm{.}21$

Using the station$-$average method, compute the missing rainfall for a storm event in which the rainfall at five gauges in the region had the following catches: $5\mathrm{.}1,5\mathrm{.}4,5\mathrm{.}3,5\mathrm{.}7,$ and $5\mathrm{.}2$ in$.$